how to fix (ground) assembly

For a part that is in an assembly, i can right click and fix its location. It seems I could not find similar function for an assembly. For example, I inserted assembly 1 into assembly 2. I would like to fix the position of assembly 1 in order to used it as a datum. What is the best way to do this?

Instead of using "fix" for assemblies, are there any issues with mating the first part to the origin, and then building up the assembly using mates? (Is this potentially a better workflow because the Assembly is then positioned at a known point in space rather than an arbitrary point in space as per the "fix" operation?)